(a) Fields of the Invention
The present invention relates to solid state imaging devices in which an imaging area with a plurality of pixels is provided on a semiconductor substrate, and to their fabrication methods.
(b) Description of Related Art
Solid state imaging devices are image sensors in which a signal received from a light receiving unit, such as a photodiode, provided in each pixel is output as an image signal, and they are classified according to signal transfer means into CCD (Charge Coupled Device)-type devices and MOS (Metal Oxide Semiconductor)-type devices.
Among them, the MOS-type solid state imaging device operates so that signals stored in a photodiode constituting a corresponding pixel are amplified by an amplifier circuit including a MOS transistor and the amplified signals are then output through interconnects such as output signal lines and horizontal signal lines to the outside of the device. The MOS-type solid state imaging device has the advantage that not only it can operate at a low voltage and read charges at a high speed but also an imaging area and a peripheral circuit can be mounted on a single chip. From these characteristics, the MOS-type solid state imaging device attracts much attention as an imaging element used in portable equipment such as a digital camera and a cellular telephone. In recent years, the MOS-type solid state imaging device has been required particularly to decrease cell size and enhance the sensitivity to incident light having a wavelength up to near-infrared area.
In a general MOS-type solid state imaging device, a single substrate is provided with: an imaging area made by arranging a plurality of pixels with respective photodiodes in rows and columns; a vertical shift register for pixel selection; a horizontal shift register for signal output; and a timing generator circuit for supplying pulses necessary for the vertical and horizontal shift registers.
In the imaging area, the photodiodes provided in the upper portion of the silicon substrate are each composed of an n-type region of a first conductivity type and a p-type region of a second conductivity type surrounding the n-type region. The upper portion of the n-type region is formed with a p+-type region, which reduces the influence of charges generated in the surface of the silicon substrate in dark conditions and thus charges generated in the photodiode are fully transferred to a floating diffusion.
In general, the amount of charges generated in the photodiode depends on the amount of absorbed incident light. In the case of a silicon substrate, light with a wavelength of 1100 nm or lower is absorbed thereinto. For example, the amount of light incident to the substrate surface decreases with depth, and the depth at which the amount of incident light decreases to half is as follows: 0.32 μm for blue light with a wavelength of 450 nm; 0.80 μm for green light with a wavelength of 550 nm; and 3.0 μm for red light with a wavelength of 700 nm. As can be seen from this, incident light with a longer wavelength is less absorbed into the substrate and thus a greater amount of the light can reach a deeper portion of the substrate. With the current state of the art, the photodiode area is formed by impurity implantation and diffusion. Thus, if the photodiode is expanded to an area capable of fully absorbing light with a long wavelength, impurities diffuse also in the horizontal direction. Therefore, in the increasingly-miniaturized solid state imaging device, it is difficult to expand the photodiode to a deeper portion of the substrate. Moreover, the p+-type layer is formed in the vicinity of the substrate surface, which causes the problem that particularly charges generated by light with a short wavelength cannot be stored in the photodiode.
Approaches for solving the above-mentioned problems include a conventional technique as disclosed in Japanese Unexamined Patent Publication No. H9-213923. FIG. 10 is a sectional view showing a photodiode unit of a conventional solid state imaging device.
Referring to FIG. 10, the photodiode includes not only a p-type region 121 and an n-type region (a charge storage region) 112 but also an n-type single-crystal semiconductor layer 171. The n-type single-crystal semiconductor layer 171 is made of germanium (Ge), silicon germanium (SiGe), gallium arsenide (GaAs), or the like having a higher absorption coefficient than silicon. With this composition, the thickness of the n-type single-crystal semiconductor layer 171 provided above the silicon substrate 111 can be smaller than that of the case where the n-type single-crystal semiconductor layer 171 is made of silicon. Alternatively, the n-type single-crystal semiconductor layer 171 is expanded in the upward direction of the silicon substrate to increase its thickness, whereby the sensitivity of the photodiode can be enhanced. Note that in FIG. 10, the reference numeral 122 denotes a p-type region, the reference numeral 113 denotes an n-type region, the reference numeral 125 denotes a p-type region, the reference numeral 141 denotes a transfer electrode, the reference numerals 132 and 135 denote insulating films, the reference numeral 134 denotes a spacer, the reference numeral 151 denotes a light shielding film, and the reference numeral 152 denotes an opening of the light shielding film.